DE10042572A1 - Electrode potential difference in standing water over sludge set to selectively oxidize or reduce impurities in sludge and water - Google Patents

Abstract

A process for reducing or preventing the admission of harmful impurities in standing water (2) over sludge (1) comprises using electrodes (4,5) in the water and sludge to from a barrier at the phase boundary between the water and sludge. Especially the potential difference between the electrodes is set to selectively oxidize or reduce impurities. A process for reducing or preventing the admission of harmful impurities in standing water (2) over sludge (1) comprises using electrodes (4,5) in the water and sludge to from a barrier at the phase boundary between the water and sludge. Especially the potential difference between the electrodes is set to selectively oxidize or reduce impurities. The process immobilizes or mobilizes impurities without allowing dissolved oxygen or hydrogen to escape from the sludge to the water. Immobilized water impurities are enriched within the sludge, and mobilized impurities are discharged from the water. The voltage applied is the redox pair specific half level potential of the impurity in question. This is derived from the U-I graph, power loss and electrical resistance between the electrodes. The working voltage applied first concentrates impurities in the sludge. The impurities are then expelled from the sludge in concentrated form and drawn from the water. The barrier between the water and sludge (2/1) is operated as a redox and/or pH barrier.

Description

The invention relates to a method for eliminating water pollutants from the aqueous Phase of a standing body of water according to the preamble of claim 1.

The process takes place in standing waters, reactors, open and closed containers and in artificial pools; Pits or trenches for concentration and concentrated decontamination of pollutants from solid matrices application.

In standing water there is excess solid matrix / water at the phase interface Transition area from the aerobic to the anoxic / anaerobic state through microbial oxygen Consumption processes for manganese (III, IV), iron (III) and sulphate reduction. The reduced manganese and iron species are transported into the aqueous phase by diffusion or convection. A large part of the phosphates is bound to iron (III) in natural systems and is used in the Iron reduction also mobilized. These biochemical reduction processes occur increased iron, manganese, phosphate concentrations in the aqueous phase, which in the Treatment of raw water to industrial or drinking water require considerable additional expenditure. So far, attempts have been made to solve the problem by going into the standing water (valley, Pre-blocking or sedimentation basin) blows air or oxygen into the hypolimnion in order to contain microbial reduction at the phase boundary between solid matrix and aqueous phase. That happens z. B. via aggregates or devices, such as ventilation pipes, pipe systems, nozzles or the like (DE 37 24 692, DE 44 35 631, EP 19509253).

A method of cleaning up stagnant water from what is in a container, such as an aquarium or the like. Above a layer of soil with plants (OS 3113021) also uses one Variant of the entry of gaseous atmospheric oxygen through a gravel filter with a Foam layer in the storage space.

The method described in WO 87/1690-A1 is based on a similar principle electrochemical oxygen production in the soil area. It's about the Improve or change the oxygen content of the water and through the evolution of gas a constant, constant circulation of the water column and mixing of the water body to achieve. In DE 197 24 137.9 A1 a method is described in which im The potential range of oxygen development is being worked on.

The problem with the previous method is that the penetration depth of the blown Oxygen in the solid matrix is only a few millimeters and the oxygen there is consumed microbially quickly. Only a very thin oxic layer is created, which immediately after Stopping ventilation reverts to an anoxic or anaerobic state. the However, microbial processes can only be influenced sustainably if they are always sufficient Oxygen is available.

The management and control of the oxygen demand has proven to be extremely problematic, because at the same time as the mechanical blowing in of air or oxygen the relatively loose Surface of the solid matrix is disturbed and swirled.

The introduction of other oxidizing agents (e.g. H ₂ O ₂ , O ₃ , peroxydisulphate, etc.) is ultimately associated with the same problems. Attempts are made to circumvent them by adding water treatment ex situ after the raw water has been withdrawn. Classic water treatment processes, such as precipitation with chemicals, adsorption on activated carbon or adsorber resins, flocculation, filtration through integrated gravel filters, etc., are used for this purpose. In DE 35 43 697 A1 z. B. Described processes and systems for removing oxidizable, dissolved metals from groundwater, in particular for domestic water supply systems. Filter devices and methods for removing iron and / or other substances from drinking water on an activated carbon filter bed are proposed in DE 34 21 113 A1. The above-mentioned conventional processes are expanded to include electrochemical methods, since substances contained in water can be reduced or oxidized in electrochemical cells. This concerns z. B. the electrochemical wastewater treatment on carbon fleece (DE 43 28 242 A1), the discharge of dissolved manganese from process waters of the paper industry via an oxidative deposition on platinum electrodes (DE 44 35 631 A1) or the entry of galvanically prepolarized, finely suspended particles into the water to be purified (EP 0 007 325).

The processes mentioned are different in electrochemical cells or reactors Size used. In this way, water purification usually takes place discontinuously, with in As a rule, waste or service water is fed to a treatment facility.

In DE 40 35 110 A1 a method is described that takes place directly in the well groundwater containing heavy metals is decontaminated by removing heavy metals from the Water flow can be fixed by ion exchange.

US Pat. No. 4,072,798 A describes a process for so-called "bioelectrical neutralization" of describe acidic waters. By adding degradable organic substances, a Forced desulfurization in the lake. If the electrode arrangement described is installed, a potential difference arises between the electrodes due to the different Oxygen conditions in the water or solid matrix horizons. This is supposed to economical electrical energy can be obtained.

Another way to reduce / prevent pollutant mobilization is to the potential water pollutants from the sediment or other solid matrices ex-situ to be removed by eluting these substances with protic or aprotic solvents will. The resulting solutions must then be dependent on the individual substance or Mixture of substances in a suitable manner (e.g. by evaporation, ion exchange, electrolysis, Precipitation, flocculation, etc.) are worked up. Is it the solid phase Sediment, this material must be obtained in a very costly manner, thickened that Pollutants are eluted and both material flows are disposed of in an environmentally friendly manner. There are too Solution approaches known that the redissolution of water pollutants (e.g. from sediments) Curb by removing the sediment from lakes and dams in an expensive manner is dredged and disposed of as hazardous waste.

The serious disadvantage of the procedure used so far and briefly described here consists in the fact that at present one mostly limits oneself to passive measures like Water treatment and / or elution of the water pollutants from the sediment as well as the To put variation of the abstraction height in the water body at the center of the activities. So far there is no continuous process for the simultaneous in-situ treatment of the water and of the sediment in waters known.

The object of the invention is to provide a method of the type mentioned at the beginning, in which there is no mixing of the sediment or any other solid matrix with the aqueous phase of the body of water comes.

According to the invention, the object is achieved by a method with the preamble of the claim 1 mentioned features achieved in that to operate the electrode assembly a Working voltage is set at which water pollutants are selectively oxidized or reduced and thus immobilized or mobilized without the dissolved oxygen or hydrogen in the body of water is released from the solid matrix, being immobilized Water pollutants accumulated in the solid matrix and mobilized water pollutants from be discharged from the body of water.

The corresponding one is advantageous for oxidizing or reducing a water pollutant redox pair-specific half-wave potential of the water pollutant set, which is determined by Recording of a U-I curve taking into account power losses and the ohmic Voltage drop between the electrodes.

The electrode potentials to be set depend on the to be immobilized or mobilizing species in the solid matrix.

In individual variants of the process, immobile or immobilized substances in the water can be used converted into its mobile redox form by appropriate polarization of the working electrode will.

The mobilized species can be removed by suitable measures (e.g. derivation in perforated Tubes or binding of the species to tissues of ion exchangers) attached to the Phase interface solid matrix / water are arranged by expansion, regeneration and Reinstallation of the tissues diverted and eliminated in external water treatment plants will.

A working electrode (AE) (approx. 3-10 cm below the solid / water phase interface) is positioned in the sediment or another solid matrix and the counter electrode (GE) is positioned in the aqueous phase (see FIG. 1) Depending on the speed of the processes taking place and the sedimentation rate in the waters, it is usually around 5-30 cm.

The working electrode is preferably perforated flat, rod-shaped or spherical. In the practical design can be nets, expanded metal, mesh and / or textile Flat structure with a conductive fiber, layer, coating or the like. Be. The mesh size depends on the type and speed of the redox processes taking place.

The counter electrode can have the same structure as the working electrode or it will a different construction chosen to allow the natural exchange of substances between Solid matrix and aqueous phase to disturb as little as possible and the living conditions of the organisms in the solid matrices only insignificantly.

After positioning the electrode system according to FIG. 1, the working electrode is polarized in such a way that oxidation and reduction reactions are initiated at the electrodes according to the redox pair-specific half-wave potentials of the water constituents (see FIG. 2). The polarity of the working electrode depends on the pollutants that are to be redox-chemically converted on or in the vicinity of this electrode. The working electrode must, for example, be anodic when converting iron (II) to iron (III), from manganese (II) to manganese (III, IV), from As (III) to As (V) or from reduced to oxidized organic carbon compounds , when converting uranium (VI) to uranium (IV) or Cr (VI) to Cr (III) are cathodically polarized. The terminal voltage is expediently determined by recording a current-voltage curve, taking into account line losses, the ohmic voltage drop between anode and cathode and the level of the desired potential at the working electrode.

For the intended operation, a minimum _{voltage U o is} necessary in order to compensate for the electrode polarization and overvoltages of the individual reactions. A working voltage U _A corresponding to the half-wave potential of the desired redox reaction is selected as the working point.

The parameters necessary for the practical use of the method are determined by measuring the U- I-curve, the specific conductivity of the water, the pH value, the oxygen consumption in the solid matrix, the heavy metal concentration (e.g. Fe, Mn) and the oxygen input as well as determined by offsetting these quantities.

In the case of the target variables manganese, iron or phosphate, the is controlled Multi-barrier system through continuous measurement of the redox potential, des Oxygen content and / or the pH value, by comparing target and actual values and changing the Control variables (switching on / off or readjustment of voltage or current).

These parameters vary depending on the type of water, the water and Sediment composition and the electrode material used.

By gradually changing the electrode potential, it becomes possible to use the various Targeted water constituents in the diffusion limit current range on the working electrode to be implemented one after the other. If the working voltage is chosen too high, anodic occurs Oxygen and cathodic hydrogen, with undesirable turbulence due to outgassing arise that have the effect of substance enrichment and immobilization in part or in full can pick up.

Through the electrochemical manipulation also species that are used as nutrients and trace substances serve, formed, transformed, fixed or released for micro- or higher organisms will. The mobile water constituents are immobilized by the electrode polarization and stored in the solid matrix so that they no longer pass into the aqueous phase can (formation of a redox barrier).

In addition to the conversions on or in the immediate vicinity of the working electrode cathodically formed hydroxide ions and anodically protons. In the case of anodic polarization of the working electrode, the protons formed are buffered by the solid matrix. the Hydroxy ions formed on the counter electrode diffuse to the phase interface Solid / water, so that a relatively high pH value (pH ≈ 10-11) develops there. It arises thus a pH gradient of ΔpH ≈ 4-6 at the phase interface. This creates a second Barrier (pH barrier) against the transfer of certain water constituents from the Solid matrix established in the aqueous phase. This applies to all harmful, nutritional, consuming and Trace substances that can be fixed under anodic conditions and / or as hydroxides in the fail or precipitate in the alkaline range.

In the case of cathodic polarization of the working electrode, hydroxide ions are analogously converted into solids components buffered. The acidic front extends to the solid / water phase interface. It all pollutants, nutrients, nutrients and trace substances are enriched and fixed, the reductive are immobilizable and / or are precipitated or co-precipitated in an acidic environment.

In addition to the price, the choice of electrode material depends largely on the type and Concentration of the water and sediment constituents as well as of the electrochemically initiated ones Reactions. Are there low contents of iron, nickel or chromium in the solid matrix permissible, then steel, high-alloy steel or nickel are conditionally suitable as electrode material. Generally, however, electrodes based on iron, chromium or nickel are not without hesitation Can be used, especially not if there is an increased sulphate and / or chloride content in the water are present. In the latter case, pitting corrosion must be accompanied by the development of chlorine be expected. Here are electrode materials with increased chlorine overvoltage and Chloride resistance necessary. This can be, for example, carbon materials, thin with Precious metal coated plastics or surface coated titanium materials (so-called. Titanodes) in the form of fleeces, mats, nets or other embodiments.

The power supply can come from conventional systems (power station electricity) as well as from alternative systems (e.g. solar energy, energy generation by wind, tides, etc.).

After concentration of certain pollutants from the pore water at the phase boundary Surface solid water can change the fixed species by reversing the polarity in a second Step are mobilized and transferred concentrated into the aqueous phase.

The advantage of the method according to the invention over known methods is that specific water constituents targeted in-situ by an applied electric field transported, concentrated and converted electrochemically or chemically. at certain voltage and current density ratios can be selected water constituents directly on the working electrode or in the immediate vicinity, in poorly soluble substances being transformed. The precipitation products settle directly on the working electrode or in the space near the electrodes and become immobile (redox barrier). In addition, through the method by means of the working electrode in the solid matrix and the counter electrode in the aqueous phase a "pH barrier" built up as a second "security level" against the microbial mobilization of certain water constituents works.

The method according to the invention is explained in more detail below with reference to two exemplary embodiments explained. The accompanying drawings show:

Fig. 1: an electrode arrangement in the electrochemical multi-barrier system

FIG. 2 shows an idealized UI curve with marking of the point voltage U ₀ and the operating points for sequential electrochemical conversion of selected substances in water

Fig. 3: Different manganese contents in the aqueous phase, directly above the phase interface

Fig. 4: different total iron content in the aqueous phase, directly above the phase boundary

Fig. 5: A remobilization of manganese from the sediment by electrical polarity reversal

Fig. 6: a remobilization of iron from the sediment by electrical polarity reversal

Embodiment 1

Original sediment ( 1 ) from a dam was installed in an experimental column according to FIG. 1. The working electrode ( 5 ) was located 3 cm below the phase interface ( 6 ). The counter electrode ( 4 ) was positioned about 7 cm above the phase interface ( 6 ) in the aqueous phase ( 2 ). The electrode ( 5 ) was connected as the anode and the electrode ( 4 ) as the cathode. Perforated stainless steel (area A = 15 cm ² ) was used as the electrode material. To determine the electrochemical process conditions, the current-voltage curve was first recorded. With a working voltage of U _A = 4 V and a current density of I / A = 0.6 mA / 15 cm ² , an oxic zone (redox barrier) was built up at the anode. B. Iron (II) is converted to iron (III) and manganese (II) to Mn (III, IV) and precipitates in concentrated form. To demonstrate the effects, the time course of the manganese (II), total iron concentration in the aqueous phase (2 ) was measured directly above the sediment ( 1 ). The measured values at U _A = 4 V and comparative values from an experiment without electrochemical polarization (U _A = 0 V) are shown in FIGS .

A selection of the gross reactions taking place at the electrodes (4 ) and ( 5 ) or in the vicinity of the electrodes using the example of iron, manganese and phosphate shows the following compilation:

anode Redox barrier

Fe ²⁺

→ Fe ³⁺

+ e ^-

Mn ²⁺

+ 2H ₂

O → MnO ₂

+ 4H ⁺

+ 2e ^-

MnO ₂

+ 2Fe ²⁺

+ 4H ⁺

→ Mn ²⁺

+ 2Fe ³⁺

+ 2H ₂

O
Fe ³⁺

+ PO ₄ ^3-

→ FePO ₄

(s)
3Mn ²⁺

+ 4H ₂

O → Mn ₃

O ₄

+ 2e ^-

+ 8H ⁺

pH barrier

Fe ³⁺

+ 3OH ^-

→ Fe (OH) ₃

(s)
H ₂

O → H ⁺

+ OH ^-

= SO ^-

+ H ⁺

→ = SOH + H ⁺

→ = SOH ₂ ⁺

(Buffering)

Cathode Redox barrier

O ₂

+ 4H ⁺

+ 4e ^-

→ 2H ₂

O
Fe ³⁺

+ e ^-

→ Fe ²⁺

pH barrier

Fe ³⁺

+ 3OH ^-

→ Fe (OH) ₃

(s)
Fe ²⁺

+ 2OH ^-

→ Fe (OH) ₂

(s)
Fe ³⁺

+ PO ^3- ₄

→ FePO ₄

(s)
Mn + 2OH ^-

→ Mn (OH) ₂

(s)

From FIGS. 3 and 4 it can be clearly seen that without polarization of the phase interface (blue curve) there is a rapid increase in the iron and manganese concentration in the aqueous phase (measured near the phase interface) as a result of the microbial reduction from immobile, higher-valued to mobile divalent ones Manganese and iron species takes place. If, on the other hand, the phase interface is anodically polarized, a redox barrier ( 8 ) is formed in the vicinity of the anode and a pH barrier (7 ) is formed between the phase interface and the cathode. B. reduced iron and manganese species formed in the depth of the sediment can get into the aqueous phase. This is achieved by oxidizing the reduced species on the anode and thereby immobilizing them. In contrast to this, in the exemplary embodiment without polarization, the manganese and iron concentration in the aqueous phase increases continuously (blue curve).

After a test duration of 36 days, a sufficiently stable oxic zone has formed (cf. FIGS. 3 and 4, phase I). At this point the electrochemical polarization was interrupted. After about 12 days in the currentless state, the oxic zone dissolves completely, manganese and iron are mobilized (phase II). This alternating mode of operation was continued in phases III-VI. Phases IV and VI followed the currentless phases (III and V).

Embodiment 2

The opposite polarization in comparison to embodiment 1 leads to a strong mobilization of the heavy metals iron and manganese (cf. FIGS. 5 and 6). By changing the polarity again, the mobilization process is ended and certain substances contained in the water are immobilized.

List of reference symbols

1

Solid matrix / sediment

2

aqueous phase

3rd

Power supply

4th

Electrode / counter electrode

5

Electrode / working electrode

6th

Phase interface

7th

pH barrier

8th

Redox barrier

Claims (20)

1. A method for reducing or preventing the entry of water pollutants into a standing body of water, having a solid matrix on which an electrode arrangement consisting of a flat electrode ( 4 ) in the water body ( 2 ) and a flat electrode ( 5 ) in the solid matrix ( 1 ), is operated to form a barrier at the phase boundary surface body of water / solids matrix (2/1), characterized in that oxidized to operate the electrode assembly (4, 5) a working voltage is set at the selectively water pollutants or reduced, and thus immobiliseirt or mobilized are released from the solid matrix (1 ) without the dissolved oxygen or hydrogen being released into the water body, whereby immobilized water pollutants are enriched in the solid matrix ( 1 ) and mobilized water pollutants are removed from the water body. 2. The method according to claim 1, characterized in that for oxidizing or reducing a water pollutant, the corresponding redox pair-specific half-wave potential of the water pollutant is set, which is determined by recording a UI curve taking into account power losses and the ohmic voltage drop between the electrodes ( 4 , 5 ) results. 3. The method according to claim 1 or 2, characterized in that the setting of the working voltage takes place so that water damage, nutrients, nutrients and trace substances in or on the solid matrix ( 1 ) first concentrated and then out of the solid matrix (1 ) in in a concentrated form and then removed from the body of water. 4. The method of claim 1 to 3, characterized in that the barrier at the phase boundary body of water / solids matrix (2/1) or is operated as a pH-barrier than redox and /. 5. The method according to claim 1 to 4, characterized in that in the solid matrix ( 1 ) a positively polarized electrode ( 5 ) and in the body of water ( 2 ) a negatively polarized electrode ( 4 ) is operated. 6. The method according to claim 1 to 4, characterized in that a negatively polarized electrode ( 5 ) and in the water body ( 2 ) a positively polarized electrode ( 4 ) is operated in the solid matrix (1). 7. The method according to claim 1 to 6, characterized in that mobilized water pollutants in the vicinity of the transition point from the solid matrix ( 1 ) to the water body ( 2 ) from the water body ( 2 ) are derived or bound. 8. The method according to claim 1 to 6, characterized in that mobile water ingredients converted to immobile species in the barrier and in the pH barrier be immobilized. 9. The method according to any one of claims 1 to 8, characterized in that for the positively or negatively polarized electrodes ( 4 , 5 ) flat, perforated electrodes made of expanded metal, carbon fleece, networks of conductive polymer material, coated films, conductive textile fabrics o The like can be used. 10. The method according to claim 9, characterized in that the electrodes ( 4 , 5 ) are stable against dissolution, decomposition or hydrogen embrittlement and do not produce any additional water pollutants directly or indirectly. 11. The method according to any one of claims 1 to 10, characterized in that the electrode in the body of water ( 2 ) is positioned so that the ohmic voltage drop is 0.01 to 42 V according to the electrolytic conductivity of the body of water ( 2). 12. The method according to claim 1 to 11, characterized in that to minimize the Energy requirement, the ohmic voltage drop is 0.01 to 5 V. 13. The method according to any one of claims 1 to 12, characterized in that the voltage or the current density can be set variably so that a certain water constituent on the electrode ( 5 ) is enriched, oxidized or reduced directly or indirectly. 14. The method according to any one of claims 1 to 13, characterized in that the working voltage is automatically controllable so that the water constituent (s) and measured variables such as redox potential, O ₂ concentration, pH value, continuously or discontinuously determined according to the target variable , this is compared with setpoint values and the target value is kept in a fixed control interval by changing the manipulated variable (s), in particular current or voltage. 15. The method according to any one of claims 1 to 14, characterized in that a direct current, a pulsed direct current, alternating voltage of variable frequency or a combination of both voltages is applied to the electrodes ( 4 , 5 ) in order to cause a specific migration of a particular water pollutant achieve and / or control the pH gradient between the electrodes ( 4 , 5 ). 16. The method according to any one of claims 1 to 15, characterized in that substances are generated or broken down at the electrodes (4 , 5 ) which can promote or inhibit the development of microorganism populations and thus influence biochemically catalyzed reactions. 17. The method according to claim 1 to 16, characterized in that on the electrodes ( 4 , 5 ) nutrients and / or trace substances are produced or eliminated in order to mobilize or immobilize a pollutant under consideration biochemically. 18. The method according to any one of claims 1 to 17, characterized in that the distance between the electrodes ( 4 , 5 ) can be selected and adjusted as desired according to the application. 19. The method according to any one of claims 1 to 18, characterized in that the polarization of the electrodes ( 4 , 5 ) is interrupted one or more times in the meantime and thus the polarization process is made periodic. 20. The method according to any one of claims 1 to 19, characterized in that the by the applied field transported species by adsorption or ion exchange on a specially for this purpose introduced membrane or a tissue is enriched.